Tungsten-halogen lamps have been utilized in the European automotive industry for a number of years and have begun to penetrate the United States automotive market during the past few years. Such lamps display several practical advantages, when compared with conventional sealed beam lamps: (1) the light emitted is whiter than that produced by the conventional lamp; (2) the tungsten-halogen lamp can be of much smaller size than the conventional lamp while producing equivalent and even greater amount of light; (3) the intensity of the illumination remains essentially constant throughout the life of the tungsten-halogen lamp; and (4) the tungsten-halogen lamp demonstrates a substantially longer working life than the conventional incandescent lamp.
Nevertheless, because the tungsten-halogen lamps operate at much higher temperatures than the standard lamp, e.g., temperatures in localized areas may range between 500.degree.-700.degree. C., glasses useful as envelopes for such lamps must be thermally stable and resist thermal deformation at those temperatures. Envelopes have been prepared from fused quartz and 96% silica glass compositions since those materials exhibit strain points far in excess of lamp operating temperatures. Disadvantageously however, those materials are quite expensive and are not readily adaptable to standard mass production techniques. Moreover, those glasses are difficult to form and lampwork and, because of their very low coefficients of thermal expansion, demand special sealing procedures to introduce the lead wires into the lamps.
Therefore, considerable research has been undertaken to provide glasses exhibiting melting and forming characteristics operable in the mass production of lamp envelopes while concurrently manifesting the physical properties required for that use. This composition research has principally involved glasses in the alkaline earth aluminosilicate system. To illustrate:
U.S. Pat. No. 3,496,401 is directed to glasses specifically formulated for use as envelopes in tungsten-iodine incandescent lamps. The glasses exhibit coefficients of thermal expansion of 30-50.times.10.sup.-7 /.degree.C., have strain points in excess of 500.degree. C., and consist essentially, expressed in weight percent on the oxide basis, of 55-70% SiO.sub.2, 13-25% Al.sub.2 O.sub.3, 10-25% alkaline earth metal oxide, and 0-10% B.sub.2 O.sub.3. Alkali metal oxides will desirably be absent, but in no event will the total thereof exceed 0.1%. The preferred glasses contain BaO and are free from B.sub.2 O.sub.3.
U.S. Pat. No. 3,798,491 is drawn to the fabrication of tungsten-halogen lamps having glass envelopes consisting essentially, expressed in weight percent on the oxide basis, of 59-70% SiO.sub.2, 10-20% Al.sub.2 O.sub.3, and 7.4-28% BaO. The glass demonstrates a coefficient of thermal expansion of 36-40.times.10.sup.-7 /.degree.C.
U.S. Pat. No. 3,978,362 is concerned with glass envelopes for tungsten-bromine lamps consisting essentially, expressed in weight percent on the oxide basis, of 58-63% SiO.sub.2, 13-16% Al.sub.2 O.sub.3, 14-21% CaO, 0-5% MgO, 0-7% BaO, with the total of CaO+MgO+BaO being at least 19%. Alkali metal oxides and B.sub.2 O.sub.3 are preferably absent. The glasses demonstrate coefficients of thermal expansion of 48-55.times.10.sup.-7 /.degree.C. and strain points in excess of 700.degree. C.
U.S. Pat. No. 4,060,423 describes glass compositions especially suitable for envelopes in tungsten-halogen lamps which manifest liquidus temperatures no greater than 1250.degree. C., strain points of at least 725.degree. C., coefficients of thermal expansion of 42-48.times.10.sup.-7 /.degree.C., and which consist essentially, expressed in weight percent on the oxide basis, of 55-68% SiO.sub.2, 15-18% Al.sub.2 O.sub.3, 7-13% CaO, and 6-16% BaO, along with minor amounts of incidental impurities, residual fluxes, and refining agents. The weight ratio CaO:BaO ranges 0.6:1-1:1. The preferred glass compositions are free from PbO, B.sub.2 O.sub.3, and alkali metal oxides.
U.S. Pat. No. 4,255,198 discloses glasses for use in tungsten-halogen lamp envelopes displaying strain points in excess of 730.degree. C., liquidus temperatures below 1200.degree. C., liquidus viscosities of at least 40,000 poises, and coefficients of thermal expansion of 43-48.times.10.sup.-7 /.degree.C. which consist essentially, expressed in weight percent on the oxide basis, of 62-64% SiO.sub.2, 14-16% Al.sub.2 O.sub.3, 10-13% CaO, and 7-9% SrO. Alkali metal oxides should be avoided and up to 5% MgO and/or BaO may be tolerated.
United States Application Ser. No. 184,764, now U.S. Pat. No. 4,302,250 filed Sept. 8, 1980 by Paul S. Danielson and entitled "Glass Envelopes for Tungsten-Halogen Lamps", includes glass compositions designed for use as glass envelopes in tungsten-halogen lamps exhibiting strain points in excess of 750.degree. C., liquidus temperatures less than 1300.degree. C., liquidus viscosities of at least 40,000 poises, viscosities of less than 1000 poises at temperatures no higher than 1520.degree. C., and coefficients of thermal expansion of 42-44.times.10.sup.-7 /.degree.C. which consist essentially, expressed in weight percent on the oxide basis, of 64-68% SiO.sub.2, 11-14% CaO, 16.5-18.5% Al.sub.2 O.sub.3, and 3-6% SrO+BaO, consisting of 0-4% SrO and 0-5% BaO. The molar ratio SrO:BaO ranges from 2:1-1:2.
European Patent Application No. 0019850, filed May 21, 1980 by Tokyo Shibaura Denki Kabushiki Kaisha, claiming priority of May 24, 1979, under the title "Halogen Incandescent Lamp" and published Dec. 10, 1980, encompasses glass compositions suitable for tungsten-halogen lamp envelopes which demonstrate strain points of at least 675.degree. C., coefficients of thermal expansion of 41-48.times.10.sup.-7 /.degree.C., and consist essentially, expressed in weight percent on the oxide basis, of 55-65% SiO.sub.2, 15-22% Al.sub.2 O.sub.3, 5-10% CaO, 6-10% MgO, and 3-6% B.sub.2 O.sub.3 plus very small amounts of accompanying oxides, residual fluxes, and residual clarifying agents. The single working example provided consisted of 57% SiO.sub.2, 20% Al.sub.2 O.sub.3, 6% CaO, 8% MgO, and 4% B.sub.2 O.sub.3, the total of those components being 95%.
U.K. Patent Application No. GB 2,060,602A, filed Oct. 8, 1980 by Carl-Zeiss Stiftung, claiming priority of Oct. 11, 1979, under the title "Alkali-free Sealing Glasses for Molybdenum" and published May 7, 1981, describes glass compositions suitable for the fabrication of tungsten-halogen lamps which exhibit transformation temperatures of 775.degree.-810.degree. C. and coefficients of thermal expansion of 46-51.times.10.sup.-7 /.degree.C. The glasses can be made colorless or having a yellow tint via the addition of CeO.sub.2 +TiO.sub.2 to the base composition. The broadest recited ranges, expressed in weight percent on the oxide basis, were:
______________________________________ SiO.sub.2 57.00-64.00 Al.sub.2 O.sub.3 12.50-16.50 ZrO.sub.2 1.00-5.50 Al.sub.2 O.sub.3 + ZrO.sub.2 15.00-19.00 CaO 11.50-19.20 BaO 0-6.50 CeO.sub.2 0-8.00 TiO.sub.2 0-4.50 CaO + BaO + CeO.sub.2 + TiO.sub.2 18.60-25.70 As.sub.2 O.sub.3 0- 0.30 ______________________________________
Unfortunately, each of the above-disclosed glasses manifests certain disadvantages, the principal one being the inherent difficulty experienced in melting and forming the compositions utilizing standard commercial mass production glass shaping techniques. Because tungsten-halogen lamp envelopes are conventionally fabricated from sections of glass tubing, the most desirable glasses will exhibit thermal stability and viscosity parameters amenable to the Vello high speed, tube drawing process.